In recent years, electric double layer capacitors and other electrochemical capacitors are receiving attention as batteries that are small and lightweight, and in which relatively large capacities can be obtained. An electric double layer capacitor does not use a chemical reaction as does an ordinary secondary battery, and features the capability of very rapid charging and discharging because it is a type of battery that directly stores electric charge on the electrodes.
By making use of such features, there are high expectation for the use of such batteries as a backup power supply for mobile equipment (small electronic equipment) and the like, an auxiliary power supply for electric automobiles and hybrid cars, and as other forms of power supplies, for example, and various forms of research are being carried out in order to improve the performance of such batteries.
An electric double layer capacitor has a basic structure in which electrolytic solution is filled by way of a separator between a pair of collectors in which a polarizable electrode layer is formed. The simplest known method for forming a polarizable electrode layer on a collector is a method of laminating these components together, but this method has a problem in that it is difficult to make the polarizable electrode layer sufficiently thin, and adequate adhesion between the collector and polarizable electrode layer cannot be obtained.
To solve the problems, the collector and polarizable electrode layer are not laminated together, but a coating solution for the polarizable electrode layer is applied to the collector, and the polarizable electrode layer is preferably formed on the collector by drying the fluid. In this case, rather than applying the coating solution for the polarizable electrode layer directly to the collector, the adhesion between the collector and the polarizable electrode layer can be greatly improved by first forming an undercoat layer as an adhesive layer on the collector, and then applying a polarizable electrode layer to the undercoat. See Japanese Patent Application Laid Open Nos. 2003-133179 and 2004-47552.
Metal in the form of a foil (metal foil) is ordinarily used as the material of the collector, and metal foil whose surface has been roughened is sometimes used for the purpose of increasing the adhesion between the collector and the polarizable electrode layer. When a metal foil whose surface has been roughened is used as a collector, the coating solution for the undercoat layer is absorbed by the unevenness of the collector, the coating solution for the undercoat layer is spread by the capillary phenomenon, and other phenomena occur that are different from the case in which the surface of the collector is not roughened when the undercoat layer is formed by coating.
For this reason, it is believed that the coating solution for the undercoat layer requires prescribed physical characteristics to be maintained in order for the undercoat layer to be correctly formed on a collector whose surface has been roughened. When these physical characteristics are not achieved, the formation area of the undercoat layer cannot be correctly controlled, nonuniformity of thickness and internal resistance increases, and other problems occur that cause the characteristics of the electrochemical capacitor to degrade.